VizAlerts allows dynamic recipients. It is a free and open-source (but not Tableau-supported) tool designed to work with Tableau Server for extremely flexible, conditional, email / SMS bursting. It will allow you to scale up to thousands of users, if you want, with no manual work. Building this as an Alert or a Conditional Subscription on Tableau Server would work, but only with manual work for each subscriber, as Nikher just mentioned. It would not scale to many.